Filtrations in the microliter range are used in various types of biochemical purifications. One example is the isolation of DNA from mixtures containing such additional species as RNA, proteins and various chemicals used in the treatment of the host cells, to obtain DNA templates for use in DNA sequencing In this and similar procedures, membrane filtration, and particularly vacuum filtration, offers the advantage of avoiding the need for precipitation and associated steps such as drying and resuspension. Membrane filtration usually involves a series of adsorption and desorption steps, by which one can isolate a species from a biochemical sample through various types of interactions with a solid phase. The membrane itself may be the adsorbing agent, or it may strictly be a filter used with a particulate adsorbent where adsorption is performed prior to filtration in a separate vessel, and the adsorbent and adsorbed material then applied to the membrane as a slurry.
Regardless of the function served by the membrane, filtrations like most biochemical procedures are most efficiently performed in batches, where numerous samples are processed simultaneously in a single piece of apparatus. This is particularly desirable when the samples are very small, such as 100 microliters or less, and where automated instrumentation is used for dispensing the various process fluids and/or monitoring and detecting the species sought to be isolated. Apparatus involving multi-well arrays in standard arrangements such as the 24-well arrangement found in Microtiter plates, or the 96-well arrangement found in larger systems, for example, are particularly useful.
Requirements of these multi-sample membrane filtration systems include the ability to provide sufficient membrane surface area for each sample to achieve full interaction between the liquid and solid phases, the ability to apply the vacuum evenly to all of the samples so that filtration occurs at a uniform rate, the ability to remove desired species from the membrane without excessive dilution by buffers or other carrier liquids, and the ability to collect the residue from each sample individually once the isolation has occurred.
Accommodating all of these needs in a single system is difficult. A typical problem is the difficulty of transfering desorbed isolate from the membrane to a collection well when the isolate is present in a very small quantity and dilution of the isolate must be minimized. A further problem is the difficulty of directing isolated species from each individual sample into a separate well while distributing the vacuum evenly below the membrane from a common vacuum source. Care must also be taken to prevent cross-contamination at other locations such as laterally through the membrane itself. Apparatus currently available to meet these needs suffer the disadvantages of a large number of parts, requiring time-consuming and complicated procedures for assembly, disassembly and cleaning.